The erythropoietin receptor (EpoR) is known for its role in erythropoiesis and is expressed in fetal liver, adult spleen, and adult bone marrow. Detection of EpoR in non-hematopoietic tissues such as brain and vascular endothelium suggests potential function for erythropoietin (Epo) beyond prolifferation, differentiation and maturation of erythroid progenitor cells. We have observed the ability of Epo to promote proliferation and block differentiation of promyoblasts. Murine promyoblasts, C2C12 cells that can differentiate into myotubes express endogenous EpoR. When grown in the presence of erythropoietin (Epo), the cells continue to proliferate and terminal differentiation is blocked and the pattern of expression of the basic-helix-loop- helix MyoD transcription family members (Myf-5 followed by MyoD and then myogenin) is altered. These results suggest that Epo may maintain a proliferative state in C2C12 cells by repressing the expression of the bHLH proteins essential for terminal differentiation. A proliferative effect of Epo is also observed on primary satalite cells from adult skeletal muscle. In promyoblasts, Epo also alters cytosolic calcium in a manner dependent on tyrosine phosphorylation. In hematopoiesis, there is a close interdependence between EpoR and GATA-1 expression. We observed that in C2C12 cells, GATA-3 transcription factor is expressed and is induced by Epo and likely influences the effect of Epo observed. Examination of the erythropoietin receptor promoter revealed three E-boxes (CANNTG) as potential binding sites for Myf-5 and other members of the Myo D (muscle) basic- helix-loop-helix transcription family members, and these markedly affects promoter activity in the promyoblast cells. That expression of EpoR may play a role in developing embryonic tissue is suggested by staining patterns in embryonic transgenic mice containing the human erythropoietin receptor promoter driving the b-galactosidase reporter gene. At embryonic day 12, transgene expression localized to the face, base of limbs and myotomes, similar to a pattern observed for the reporter gene driven by the promoter for the early muscle transcription factor, Myf-5. These observations suggest that in addition to erythropoiesis, erythropoietin may play a role in development of non- hematopoietic tissue in promoting proliferation during expansion and differentiation of select cell types.